In helical scanning magnetic tape recording systems, tracks are laid diagonally across the magnetic tape medium by means of a rotating cylindrical head drum having at least one magnetic read/write head embedded in the drum surface and magnetic tape wrapped helically around the head. The angle of the track is fixed by the geometry of the system and the relative tape and head speeds. The placement of the longitudinal component of the track is determined by the wrap angle of the tape around the head and the timing of head activation for recording signals onto the tape. For recording, it is imperative to energize the write head over an accurate angular range of the head drum rotation to place the track precisely on the tape. Similarly, for reading, it is necessary to activate the reading circuits over an accurate angular range of the head drum rotation in order to sense the entire length of the track recorded on the tape. To accurately write and read data, then, it is necessary to calibrate the system for the angular information. In high density helical scan recording, this calibration must be performed at a much higher level of accuracy than that achieved in prior art systems which relied primarily on mechanical specifications.
Prior art calibration of helical scan head drum angles typically utilized an electrical encoder such as a Hall sensor or coil fixed to the stator of the drum motor to sense the position of the rotor of the drum motor using a magnet or a series of magnetic poles fixed to the rotor. The output of the encoder is amplified and transmitted through electrical circuits to generate track timing. Using a calibration tape or other reference device, an adjustable element (for example, a trimming potentiometer) in the electrical circuit is manually controlled by a technician to adjust the circuit timing to match the track timing. In a multiple-head drum, this step must be repeated for each head.
Some prior art helical scanning head drum calibration systems include two encoders. One encoder generates a "pulse generator" (PG) signal being a single detectable feature (typically a pulse) for every complete revolution of the head drum. The second encoder generates a "frequency generator" (FG) signal which is a series of detectable features (typically a sine wave) having a predetermined number of cycles for every complete revolution of the head drum (for example 24 cycles per revolution). The FG encoder normally provides higher accuracy angular information than the PG encoder. Therefore, the typical procedure is to first detect the PG encoder signal to identify a particular approximate angle on the head drum and to then detect the next feature of the FG encoder signal to add greater accuracy and resolution to the angular information. Such calibration systems have the disadvantage of circuit complexity resulting from circuit designs necessary to implement the aforesaid procedure.